Search code examples
c++cnoreturn

Why noreturn/__builtin_unreachable prevents tail call optimization

I have come to fact that all major compilers will not do tail call optimization if a called function does not return (i.e. marked as _Noreturn/[[noreturn]] or there is a __builtin_unreachable() after the call). Is this an intended behavior and not a missed optimization, and if so why?

Example 1:

#ifndef __cplusplus
#define NORETURN _Noreturn
#else
#define NORETURN [[noreturn]]
#endif

void canret(void);
NORETURN void noret(void);

void foo(void) { canret(); }
void bar(void) { noret(); }

C: https://godbolt.org/z/pJfEe- C++: https://godbolt.org/z/-4c78K

Example 2:

#ifdef _MSC_VER
#define UNREACHABLE __assume(0)
#else
#define UNREACHABLE __builtin_unreachable()
#endif

void f(void);

void foo(void) { f(); }
void bar(void) { f(); UNREACHABLE; }

https://godbolt.org/z/PFhWKR

Solution

  • It's intentional, though perhaps controversial since it can seriously harm stack usage properties; for this reason I've even resorted to tricking the compiler to think a function that can't return can. The reasoning is that many noreturn functions are abort-like (or even call abort), and that it's likely someone running a debugger wants to be able to see where the call happened from -- information which would be lost by a tail call.

    Citations: